1. Field of the Invention
The present invention relates to a hybrid drive system for a motor vehicle having at least one internal combustion engine and at least one electric motor driving at least one wheel of the motor vehicle.
2. Description of Related Art
Against the background of rising crude oil prices and the looming change in the earth's climate, there is a constantly growing demand for the most fuel-efficient motor vehicles.
One promising approach is to use motor vehicles having hybrid drive systems. In hybrid drive systems, another motor using a different form of energy to drive the vehicle is used in addition to the normal internal combustion engine. Electric motors have become popular in practice for this purpose.
Due to the use of additional motors, it is possible on the one hand to operate the internal combustion engine mostly in a particularly energy-efficient mode. The driving power supplied by the internal combustion engine and not utilized to drive the motor vehicle at a given time may be stored temporarily in an energy storage such as a battery. The energy stored temporarily in this way may be used at a later time to drive the motor vehicle. This makes it possible to reduce the load on the internal combustion engine or turn it off completely. It is also possible to design the internal combustion engine to have smaller dimensions, if necessary. Furthermore, it is possible to convert the kinetic energy of the motor vehicle into electrical energy during deceleration of the vehicle and to store this electrical energy temporarily in the battery. The braking energy is thereby not “lost.”
On the basis of these (and additional) effects, motor vehicles using hybrid drive systems are particularly fuel-efficient, in particular when the motor vehicle is operated in city traffic.
Since the technology is still relatively new, there are a great many as yet unsolved detail problems which have so far prevented widespread success of hybrid drive systems.
For example, so-called axle-split hybrid drives (the drive motors act on different drive axles) are problematic. In these systems, one axle of the vehicle is driven by a conventional internal combustion engine, while the electric motors of the hybrid drive act on a second axle. In such axle-split hybrid drives, the electric motor cannot cover the entire vehicle velocity range. On the one hand, the maximum possible speed of the electric motor is limited on the upside to prevent damage to the electric motor. However, if the electric motor is operated at low rotational speeds, this results in unfavorable efficiency of the electric motor, which is of course undesirable. Designing the operating point of the electric motor therefore represents a compromise which causes problems at both ends of the velocity range of the vehicle.
To be less restricted by the upper speed limit of the electric motor, published European patent document EP 0 224 144 A1 has already proposed allowing the mechanical connection between the drive axle and the electric motor to be achieved by a clutch. The efficiency of the electric motor in the low velocity range of the motor vehicle may be increased in this way. At the same time, damage to the electric motor at high vehicle velocities may be prevented by disengaging the clutch.
However, the hybrid drive systems known so far still have disadvantages.